Modify Header Value (HTTP Headers) - Modify Header Value (HTTP Headers) is a browser extension that allows you to add, modify or remove a header for any request to your preferred domains.
Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
Sky Live HTTP Headers - Sky Live HTTP Headers is an open-source browser extension that visually displays HTTP requests and response headers from every website you visit.
HTTP Toolkit - Beautiful, cross-platform & open-source tools to debug, test & build with HTTP(S). One-click setup for browsers, servers, Android, CLI tools, scripts and more.
WebSniffer - WebSniffer is an online tool to outlook HTTP request and response headers.
Weer - A HTTP protocol debugger with Chrome DevTools frontend interface